Method for the preparation of perfluoroolefins



Un t States Patent O a Claims. 01. 260 6533) The present invention relates to a method for the preparation of perfiuoroolefins, and more particularly to a method for the preparation of perfiuoroolefins by pyrolysis of perfluorocarbon acids.

It was heretofore known, US. Patent 2,601,536, issued June 24, 1952 to James D. LaZerte, and U.S. Patent 2,668,864, issued February 9, 1954 to Lyle J. Hals,

Thomas S. Reid and George H. Smith, that alkali metal salts of perfiuorocarboxylic acids herein also referred to as perfiuorocarbon acids, couldlbe decarboxylated and defluorinated to form perfiuoroolefins. It is stated in these patents that the alkali metal salts are unique in their behavior when heated to temperatures at which this reaction occurs. Other perfiuorocarboxylic acid derivatives and the. acid itself are said not to decarboxylate or to only slightly decarboxylate to form the perfiuoroolefins.

It is an object of the present invention toprepare perfiuoroolefins by pyrolysis of aliphatic perfluorocarboxylic acids. It is another object of'the present invention to prepare perfiuoroolefins in high yields from perfluorocarboxylic acids.

The objects of the present invention are accomplished by a process which comprises passing a substantially anhydrous mixture of a perfluorocarboxylic acid having the formula F-C,,F ,,--COOH, where n is an integer from 2. to 11 and an inert gaseous diluent through a reaction zone maintained at a temperature of 400 C. to 650 C. and recovering a perfiuorinated olefin having the formula n2 2(n2) 2' In accordance with the present invention it was discovered, contrary to the disclosures of the prior art, that perfiuoroolefins could be obtained in high yields by the decarboxylation of perfluorocarbon acids. It was found, however, that the decarboxylation occurred in significant and commercially attractive yields only if the perfiuorocarbon acid was decarboxylated in vaporized form in the presence of an excess of gaseous diluent, preferably at hot zone contact times not exceeding five seconds. Diluents suitable in the process of the present invention are nitrogen, carbon dioxide and similar inert gases. The gaseous diluents are preferably employed in large excess, particularly suitable mixtures of diluent and vaporized acid are mixtures which contain less than by volume the vaporized acid.

The decarboxylation may be carried out by various means. In the preferred method the mixture of acid and diluent is passed through a reaction tube heated to the desired reaction temperature. The resulting reaction products are condensed in cold traps and the perfiuoroolefin is recovered and separated by means of distillation. The reaction tube may bemade of stainless steel,- or steel lined with silver, platinum or any other corrosion resistant metal. If desired, the tube may be filled with refractory oxides or other inert materials to achieve better heat transfer to the reaction mixture. The decarboxylation is generally carried out at atmospheric pressure, although higher or lower pressures may be employed, except the pressure may not exceed the critical pressure of 2,926,203 Patented Feb. 23, 1:960

ice.

the acid at "reaction temperatures inorder to avoid con: densation of the acid.

The invention is further illustrated by the following examples.

Example I- Into-a- Monel tube, 222cm. long and having a 1:59 cm. insidediameter, heated to a temperature of 650563., was passed a mixture of 3.5 mL/min. of vaporized "perfluoropropionic acid and 500 mL/min. of carbon dioxide. The contact time of the gases in the heated Mariel tube was 3 seconds. The hot reaction gases obtained were condensed in a liquid nitrogen 'coldtrap." spectrographic analysis of the fluorocarbon products shows that 83% was tetrafiuoroethylerii The example was repeated without the use of a diluent. Only traces of tetrafluoroethylene were found in the reaction product.

Example 11 Following the procedure of Example I, perfluorobutyric acid was charged to the heated Monel tube at the rate of 2 ml. per minute and mixed with carbon dioxide at the rate of 500 ml/min. Analysis of the resulting fluorocarbon products showed an 86% yield of perfiuoropropylene.

Example III Into a borosilicate glass tube, having a length of 12 in. and an inside diameter of 0.7 in., and containing in the center a plug of glass wool, heated to a temperature of 620 C., there was charged 3.4 g. of perfiuorooctanoic acid over a period of 12 min. in a stream of nitrogen passed through the tube at the rate 0155,00 ml./min. The resulting reaction products were condensed in liquid nitrogen traps. There was obtained on distillation of the products 1.8 g. of perfiuoroheptene-l corresponding to a yield of based on unreacted acid recovered. The example was repeated using carbon dioxide as the diluent. There was obtained 1.2 g. of perfiuoroheptene-l corresponding to a yield of 89% on the basis of uning the process of the present invention, and various modifications will occur to those skilled in the art. The process, for example, is'not limited by the structure of the perfluorocarbon acid employed. It is thus possible to prepare branched as well as linear perfiuoroolefins from the corresponding perfluorocarboxylic acids. The process of the present invention is furthermore not limited to the number of carbon atoms in the perfluorocarboxylic acid. However, it is preferred to apply the Process of the present invention to perfiuorocarboxylic acids having less than 12 carbon atoms, since the chemical stability of perfluorocarboxylic acids having more than 12 carbon atoms is lower and results in a larger amount of decomposition of the acid and hence a lower yield. The process of the present invention is also suitable for the decal"- boxylation of dicarboxylic perfiuorocarbon acids. In addition to the perfluorocarboxylic acids illustrated in the examples, there may be employed in the process of the present invention, perfiuorocarboxylic acids such as perfiuoroisobutyric acid, perfluoropentanoie acid, perfiuorosebacic acid, perfiuorohexanoic' acid, perfluoroheptanoic acid and perfluorodecanoic acid. In addition to the completely fluorinated acids employed in the decarboxylication of the present invention there may be employed omega-hydroperfluorocarboxylic acids, which are essentially identical in their physical and chemical properties Examples of such acids are 4-hydrohexafiuorobutanoic acid, 7-hydrododecafluoroheptanoic acid, etc. I v

The perfiuoroolefins prepared by the process of the present invention are highly useful monomers which can be employed in the formation of perfluorinated'pblym ers having outstanding physical and chemical properties; particularly useful polymers prepared from the monomers of the present invention are polytetrafiuoroethylene and copolymers of .tetrafiuoro'ethylene' with' perfiuorinated monomers such as perfluoropropylene, perfluorobutene, perfluoropentene, etc.

I claim:

1. A process for the preparation of perfluoroolefins which comprises passing through a hot zone maintained ata temperature of 400 to 650 C. a perfluorocarboxylic acid having the formula FC,,F COOH where n is an integer of 2 to 11 in an inert gaseous diluent, said perfiuorocarboxylic acid being employed in a concentration of not greater than 10 volume percent of said diluent, and re- 16 covering a perfluoroolefin having the formula V 4 2'. The'process as sef'forth in claim 1 wherein the perfluorocarboxylic acid is maintained in the hot zone for a contact time of less than 5 seconds.

3. The process as set forth in claim 2 wherein the per- 5 fluorocarboxylic acid is perfluoropropionic acid.

4. The process as set forth in claim 2 wherein the perfluorocarboxylic acid is perfluorobutyric acid.

5. The process as set forth in claim 2 wherein the perfiuorocarboxylic acid is perfluorooctanoic acid.

,6. The process as set forth in claim 2 wherein the diluent is nitrogen. I v I 7.' The process as set forth in claim 2 wherein thediluent is carbon dioxide.

La Zerte June 24, 1952 Hals et al. Feb. 9, 1954 

1. A PROCESS FOR THE PREPARATION OF PERFLUOROOLEFINS WHICH COMPRISES PASSING THROUGH A HOT ZONE MAINTAINED AT A TEMPERATURE OF 400 TO 650*C. A PERFLUOROCARBOXYLIC ACID HAVING THE FORMULA F-CNF2NCOOH WHERE N IS AN INTEGER OF 2 TO 11 IN AN INERT GASEOUS DILUENT, SAID PERFLUOROCARBOXYLIC ACID BEING EMPLOYED IN A CONCENTRATION OF NOT GREATER THAN 10 VOLUME PERCENT OF SAID DILUENT, AND RECOVERING A PERFLUOROOLEFIN HAVIGN THE FORMULA 